Skip find_package for already present targets

[GregTheMadMonk]

For #231.

Didn't properly test yet, and I don't know what to do with msgpack-c and PkgConfig, if anything needs to be done for them

[liuzicheng1987]

@GregTheMadMonk , thanks for the initiative. Let me know when this is finished (it appears that this is still a draft).

[xsun2001]

I would be appreciate if the same support is provided to toml format for better CMake/CPM intergration.

if (REFLECTCPP_TOML)
    list(APPEND REFLECT_CPP_SOURCES
        src/reflectcpp_toml.cpp
    )
    if (TARGET tomlplusplus::tomlplusplus)
		target_link_libraries(reflectcpp PRIVATE tomlplusplus::tomlplusplus)
    else()
		if (MSVC)
	        target_link_libraries(reflectcpp PRIVATE "${VCPKG_INSTALLED_DIR}/${VCPKG_TARGET_TRIPLET}/lib/tomlplusplus${CMAKE_STATIC_LIBRARY_SUFFIX}")
	    else ()
	        target_link_libraries(reflectcpp PRIVATE "${VCPKG_INSTALLED_DIR}/${VCPKG_TARGET_TRIPLET}/lib/libtomlplusplus${CMAKE_STATIC_LIBRARY_SUFFIX}")
	    endif ()
	endif()
endif()

[liuzicheng1987]

@GregTheMadMonk , what is status on this PR? It is still marked as a draft...is that on purpose? Do you want me to review and then potentially merge it?

[GregTheMadMonk]

@liuzicheng1987 yes, sorry, I was sidetracked by other tasks and kind of pushed this to the side.

Here I've rebased the branch to match your current main, added it for (I hope) all possible dependencies (@xsun2001 including tomlplusplus) and tried to test the libraries and how they work with this patch. Aside from the following, everything worked fine as far as I can tell:

1. CMakeLists.txt in yyjson repo only provides a yyjson target, yyjson::yyjson needs to be provided manually as an alias after importing yyjson project. Similar situation with bson_static, bson_shared and flatbuffers. I've decided to leave it as is, as declaring an ALIAS library is not hard and this helps keep things uniform with what is expected from found packages
2. msgpack (I used cpp_master branch) throws a no such file or directory error on #include <msgpack.h> for some reason
3. tinycbor does not provide a CMakeLists.txt at all. I made it so that if a tinycbor target is present in the project (probably declared manually by a person who uses tinycbor with cmake), it is used instead of searching vcpkg paths. Didn't test it yet
4. Didn't test benchmark and simdjson

Additionally, I've made it so that install and other install code is only called if reflectcpp is a toplevel project because it threw an error trying to declare exports for dependencies too. Which is probably a correct thing to do anyway, since a local dependency of some project should not be installed system-wide

IDK if I should mark it as a regular PR or leave it a draft for now. It'd be great if someone else tested and verified that it doesn't break builds for them too.

edit: Removed the "Draft" mark, since I don't think there's anything else to add here except maybe get msgpack working and at least have someone else verify it works for them too.